1. Field of the Invention
The present invention relates to free-space optical communication systems, and more specifically, it relates to improvements in the dynamic range of laser beam tracking in free-space optical communication systems.
2. Description of Related Art
Free-space optical communications systems are dependent upon the accuracy and stability of the pointing of a collimated laser beam. High stability must be maintained in spite of mounting platform vibrations, thermal drift, and especially atmospheric turbulence and scintillation which causes large and rapid fluctuations in beam position and intensity. A beam position sensing and feedback control system for stabilization is required and has been implemented in prior art by several means including position sensing detectors, quadrant cells, cameras of various types, and elaborate adaptive optics systems. One shortfall in the prior art is a lack of dynamic range, resulting in unacceptable compromises in pointing precision and/or control bandwidth.
Ubiquitous commercial solutions (e.g., Ontrak Photonics, Inc.; Hamamatsu Corp.; Melles Griot) use DC-based approaches to signal detection and processing. This provides no rejection of ambient light or other interfering sources and is subject to DC drift and 1/f noise.
Many approaches have used analog divider chips (e.g., Ontrak Photonics, Inc.) that are subject to offset errors, resulting in large errors at low signal levels. Dynamic range is rarely more than 20 to 25 dB optical.
The method reported by Narayanan, et. al. [Ref. 4] can be augmented with the AGC chain of the present invention to dramatically enhance dynamic range.
An improvement in dynamic range that provides pointing precision and/or control bandwidth is desirable. The present invention provides such an improvement.